Pressure relief valves are used in various hydraulic applications to prevent the accumulation of excessive hydraulic pressure that might otherwise damage a hydraulic machine or its components. Upon actuation, hydraulic pressure relief valves open to permit hydraulic fluid to pass through the valve. The hydraulic fluid is often routed to a sump, or other reservoir, for recirculation through the hydraulic system. The valves are typically adjustable to permit the selection of a specific pressure at which the valve will actuate to relieve pressure.
One particular application where pressure relief valves are used is hydrostatic drive units that propel various machines such as tractors, bulldozers, skid steers and other heavy equipment. The hydrostatic drive units may also provide motive forces for various implements used on these machines. In such applications, it is important that the pressure relief valves be correctly tuned to relieve hydraulic pressure at a predetermined pressure to ensure reliable operation of the machine. If the relief pressure is set too high, the machine can be damaged; if the pressure is set too low, oil is dumped too soon and the machine can stall under load. In typical applications, multiple valves may be used with a single pump to provide the motive force for various functions of the machine. For example, a drive system configured to provide left, right, forward and reverse movement of a machine may typically utilize four valves per pump, one valve for each of the aforementioned motions. If each of the four valves is not correctly tuned, the drive system will not function properly.
Generally, it is impractical to tune the pressure relief valves while they are installed on the machine. For example, a hydrostatic drive system may be located in an area of the machine that is not readily accessible, such as beneath an operator cab. Furthermore, when the valves are installed on the machine, it is difficult to obtain an accurate determination of the pressure at which the valve actuates, or “cracks”, to relieve the pressure because the drive system includes a cut-off mechanism that senses pressure in the pump and limits stroke of the pump piston to control the pressure. Furthermore, the pumps are typically capable of supplying a flow of hydraulic fluid that is much greater than the through-flow capacity of the valves. When the valves are actuated on the machine, it is often difficult to observe the associated drop in pressure if the pump piston has not destroked. It is inadvisable to set the pressure relief valves blindly by adjusting them to a fully closed position and then backing the adjustment off an arbitrary amount because such adjustment may set the actuation pressure too high, creating excessive pressures which may damage the equipment.
Accordingly, there is a need for apparatus and methods for tuning pressure relief valves prior to installation in a machine and which overcomes drawbacks of the prior art, such as those discussed above.